Breast cancer is one of the most common malignant tumors in women, with a high incidence rate and invasiveness, but the course of progression is slow. “Chinese Breast Disease Investigation Report” issued in Beijing on 1 Feb. 2010 by Chinese Population Association showed that the death rate of breast cancer in Chinese urban areas has increased by 38.91%, and that breast cancer has become the greatest threat to women's health. At present, there are at least 156 drugs for breast cancer under development or on the market, in which 68% are targeted drugs. A number of researchers have shown that tumor is related to cell cycle abnormalities, mutations of mitotic signaling proteins and defects of anti-mitotic signaling proteins in tumor cells leading to proliferation disorders. Meanwhile, most of the tumors have genomic instability (GIN) and chromosome complement instability (CIN), and these three basic cell cycle defects are all induced directly or indirectly by out of control of cyclin dependent kinases (CDKs). Cyclin Dependent Kinase (CDK) has become an increasingly popular target.
Currently, many first- and second-generation CDK inhibitors have been developed. The most noticed second-generation drug includes a CDK4&6 inhibitor PD-0332991, which was jointly developed by Pfizer and Onyx. It inhibits the phosphorylation of Rb by inhibiting the activity of CDK4&6, enables the E2F-Rb complex to be detained in the cytoplasm, and blocks initiation of the cell cycle. The results of a clinical trial (NCT00721409) showed that the progression-free survival (PFS) of patients treated with letrozole alone was 7.5 months, whereas the progression-free survival of patients subjected to combined treatment of letrozole and PD-0332991 was extended to 26.1 months. This remarkable advantage has received widespread attention. At the beginning of 2013, the FDA considered that it might be a groundbreaking anticancer drug after reviewing the mid-term result of the drug.
International Patent Application Publication WO2014183520 discloses CDK4&6 inhibitors similar to PD-0332991 in structure, with significant inhibitory activity and high selectivity for CDK4&6, comprising the following compound:

However, this compound has poor solubility, and cannot be used directly as a drug. There is a need to find a pharmaceutically acceptable form, which makes it possible to enhance its solubility and bioavailability.
On the other hand, it is known to those skilled in the art that the crystal structure of the pharmaceutically active ingredient often affects the chemical stability of the drug. Different crystallization conditions and storage conditions can lead to changes in the crystal structure of the compound, and sometimes the accompanying production of other crystal forms. In general, an amorphous drug product does not have a regular crystal structure, and often has other defects such as poor product stability, smaller particle size, difficult filtration, easy agglomeration, and poor liquidity. Thus, it is necessary to improve the various properties of the above product. There is a need to identify a new crystal form with high purity and good chemical stability.